the colored light transmitted perpendicularly through a
thin plate of opal undergoes no change when the gem is turned round in
its own plane. This appears to prove that the alternate states are not
related to one another as twin crystals. More probably the alternate
strata are of air, as in decomposed glass. The brilliancy of opals is
said to be readily affected by atmospheric conditions.

* * * * *

A NEW CATALOGUE OF VALUABLE PAPERS

Contained in SCIENTIFIC AMERICAN SUPPLEMENT during the past
ten years, sent _free of charge_ to any address. MUNN & CO., 361
Broadway, New York.

* * * * *

THE SCIENTIFIC AMERICAN

ARCHITECTS AND BUILDERS EDITION.

$2.50 a Year. Single Copies, 25 cts.

This is a Special Edition of the SCIENTIFIC AMERICAN, issued
monthly--on the first day of the month. Each number contains about
forty large quarto pages, equal to about two hundred ordinary book
pages, forming, practically, a large and splendid MAGAZINE OF
ARCHITECTURE, richly adorned with _elegant plates in colors_ and with
fine engravings, illustrating the most interesting examples of modern
Architectural Construction and allied subjects.

A special feature is the presentation in each number of a variety of
the latest and best plans for private residences, city and country,
including those of very moderate cost as well as the more expensive.
Drawings in perspective and in color are given, together with full
Plans, Specifications, Costs, Bills of Estimate, and Sheets of
Details.

No other building paper contains so many plans, details, and
specifications regularly presented as the SCIENTIFIC AMERICAN.
Hundreds of dwellings have already been erected on the various plans
we have issued during the past year, and many others are in process of
construction.

Architects, Builders, and Owners will find this work valuable in
furnishing fresh and useful suggestions. All who contemplate building
or improving homes, or erecting structures of any kind, have before
them in this work an almost _endless series of the latest and best
examples_ from which to make selections, thus saving time and money.

Many other subjects, including Sewerage, Piping, Lighting, Warming,
Ventilating, Decorating, Laying out of Grounds, etc., are illustrated.
An extensive Compendium of Manufacturers' Announcements is also given,
in which the most reliable and approved Building Materials, Goods,